Various metal parts are typically coated with polymers for various reasons. The polymer coating, for instance, can prevent the underlying metal from corroding, especially when the metal part is to be used in a corrosive environment, such as an acidic environment or an alkaline environment.
In one embodiment, the metal parts are powder coated. During powder coating, a hot metal part is dipped into a fluidized bed. The fluidized bed contains fluidized polymer particles that stick to the hot metal part and form a coating.
The polymers that are used to coat the metal parts can vary depending upon the particular application. For example, polyester resins are typically used for outdoor applications. Polyester resins, for instance, have inherent UV stability. Polyester resins can also be formulated so that they will adhere to metal. Polyesters, however, are hydrolytically unstable which limits their use. For instance, polyesters are typically not well suited for use in environments where the polymer must have hot water resistance or chemical resistance, especially resistance to alkaline materials.
For example, many metal coated parts are used in applications where the part is periodically or continuously exposed to hot water, steam and/or corrosive chemicals. Coatings applied to dishwater racks, for instance, need to be stable in hot water environments and in alkaline environments, since dishwater detergents are typically very alkaline. Thus, in the past, polymer coatings containing primarily polyamide 11 or polyamide 12 have been proposed for use as a metal coating in many hot water environments and corrosive environments. Polyamide 11 and polyamide 12, for instance, have adequate mechanical properties such as abrasion resistance and impact strength, and are chemically inert to hydrocarbons, mineral acids, and bases. The use of polyamide 11 and polyamide 12 coatings for metal parts, for instance, are described in U.S. Patent Publication No. 2010/0113670, which is incorporated herein by reference. Resins that contain primarily polyamide 11 and/or polyamide 12, however, are relatively expensive.
Another type of polymer that has excellent mechanical properties and excellent chemical resistance properties are polyoxymethylene polymers. For example, polyoxymethylene polymers do not mechanically or chemically degrade when exposed to hot water, steam, or alkaline compounds. Polyoxymethylene polymers, however, have not been widely used to coat metal parts, since the polymers exhibit poor adhesion to metal surfaces. In addition, the polymers have high stiffness and high shrinkage, which can result in cracking. Once a coating cracks, for instance, the coating has a tendency to flake off easily. Those skilled in the art have attempted to address this problem by combining polyoxymethylene polymers with an assortment of additives. These formulations, however, have met with little success.
In view of the above, a need exists for a polymer composition containing a polyoxymethylene polymer that can be used to powder coat metal substrates.